I. The Field of the Invention
The present invention generally relates to the field of medical devices. More specifically, the present invention relates to methods, systems, and devices for manufacturing a self-expanding medical device.
II. Related Technology
The use of intravascular devices to treat cardiovascular diseases is well known in the field of medicine. The need for a greater variety of devices to address different types of circumstances has grown tremendously as the techniques for using intravascular devices has progressed. One type of intravascular device is a stent or scaffold. Stents and scaffolds are generally cylindrically shaped intravascular devices that are placed within an artery (or other vessel within the body) to hold it open. The device can be used to reduce the likelihood of restenosis or recurrence of the blocking of a blood vessel and can be placed within an artery on a permanent basis, such as a stent, or a temporary basis, such as a scaffold. In some circumstances, a stent or scaffold can be used as the primary treatment device where it is expanded to dilate a stenosis and left in place.
A variety of stent and scaffold designs have been developed. Examples include coiled wires in a variety of patterns that are expanded after being placed within a vessel on a balloon catheter, helically wound coiled springs manufactured from expandable heat sensitive metals, stents or scaffolds shaped in zig-zag patterns, and self-expanding stents and scaffolds inserted in a compressed state for deployment in a body lumen.
Stents and scaffolds can have various features. For instance, a stent or scaffold can have a tubular shape formed from a plurality of interconnected struts and/or legs that can form a series of interconnected rings. In the expanded condition, the stent or scaffold can have a cylindrical shape to expand in an artery. One material for manufacturing self-expanding stents or scaffolds is nitinol, an alloy of nickel and titanium.
The conventional approach to manufacture a self-expanding stent or scaffold is to begin by laser cutting the design of the stent or scaffold from a tube having a diameter that is approximately equal to the desired diameter of the compressed (i.e., unexpanded) stent or scaffold. The tube is then deburred to clean any imperfections due to the cutting. Once the tube has been deburred, the tube is then expanded to the desired diameter, which is the diameter the stent will maintain when left within a body vessel. The tube is then heat set at the desired expanded diameter to maintain the tube at that diameter.
Conventionally, expanding the stent or scaffold to the desired expanded diameter requires an iterative process: The tube is positioned on a mandrel having a diameter that is slightly larger than the diameter of the compressed tube, thereby expanding the tube. Heat is applied to the tube while the tube is on the mandrel to heat set the tube at the new diameter. The tube and mandrel are allowed to cool to complete the heat setting, and the tube is then removed from the mandrel. This process is then repeated with a slightly larger mandrel to expand the tube further. This iterative process of expanding the tube a little at a time is repeated until the desired expanded diameter is attained.
Although the conventional manufacturing approach discussed above generally yields acceptable self expanding medical devices, the approach has some shortcomings. For example, it is cumbersome and time consuming due, in large part, to the iterative heating and cooling processes. In addition, a significant amount of energy is used by heating and reheating the medical device and the mandrel during each iteration. Another shortcoming is that, in many instances, cracks are induced in the stent or scaffold during conventional manufacturing due to undesired torque, tension, expansion, and/or compression.